Auxins biosynthesis physiological role and mechanism

BIOSYNTHESIS, TRANSLOCATION, PHYSIOLOGICAL ROLE AND MECHANISM OF AUXIN

1. BIOSYNTHESIS, TRANSLOCATION, PHYSIOLOGICAL ROLE AND MECHANISM OF AUXIN FROM B. PAVAN KUMAR NAIK 1 st YEAR P h .D DEPARTMENT OF HORTICULTURE FACULTY OF AGRICULTURE ANNAMALAI UNIVERSITY

2. Plants require light, water, oxygen, minerals and other nutrients for their growth and development. Apart from these external requirements, plants also depend on certain organic compounds to signal, regulate and control the growth of plants. These are collectively called as Plant Growth Regulators or Plant Growth Hormones. INTRODUCTION

3. Plant growth regulators or phytohormones are organic substances produced naturally in higher plants, controlling growth or other physiological functions at a site remote from its place of production and active in minute amounts. Thimmann (1948) proposed the term Phytohormone as these hormones are synthesized in plants. Plant growth regulators include auxins , gibberellins, cytokinins , ethylene, growth retardants and growth inhibitors. Auxins are the hormones first discovered in plants and later gibberellins and cytokinins were also discovered. PLANT GROWTH REGULATORS

4. Plant growth regulators can be defined as organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes. They are signal molecules, produced within plants that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress and tolerance to reproductive development. Cont …

5. They are also referred to as plant growth regulators or phytohormones . Plant growth hormones are organic compounds which are either produced naturally within the plants or are synthesized in laboratories. Generally, there are five types of plant hormones namely, Auxins , Gibberellins (GAs), Cytokinins , Abscisic acid (ABA) and Ethylene. In addition to these, there are more derivative compounds, both natural and synthetic, which also act as plant growth regulators. Cont …

6. Auxins were the first class of plant hormones to be identified. The term auxin is derived from the Greek word auxein which means to grow. All those growth regulating organic compounds which are produced at the tips of roots and stem as a result of metabolism and transported to the region of elongation causing elongation of cells are called Auxins . Both natural and synthetic Auxins are known and all have similar effect on plant growth and development. Thimann (1948) defined an auxin as “an organic substance which promotes growth along the longitudinal axis when applied in low concentrations to shoots of plants freed as far possible from their own inherent growth promoting substances” AUXINS

7. Auxins passes from shoot tip to the region of elongation and its movement is basipetal (from apex towards base) in stem but acropetal ( from base towards apex) in roots . Auxin helps in elongation of both shoots and roots. However, optimum for the two is quite different (10 ppm for stem and 0.0001ppm for root ). Auxin biosynthesis occurs in shoot apices, leaf primordial and developing seeds from amino acid tryptophan in the presence of Zn++ ion. The most important member of the auxin family is indoleacetic acid (IAA), which generates the majority of auxin effects in intact plants, and is the most potent native auxin . And as native auxin , its stability is controlled in many ways in plants, from synthesis, through possible conjugation to degradation of its molecules, always according to the requirements of the situation. Cont …

8. Naturally occurring (endogenous) auxins in plants include indole-3-acetic acid, 4- chloroindole-3-acetic acid, phenylacetic acid, indole-3-buyyric acid and indole-3-propionic acid. Synthetic auxins include 1-naphthaleneacetic acid, 2,4- D(2,4- dichlorophenoxyacetic acid) and IBA : Indole Butyric Acid NAA : Naphthalene Acetic acid MENA : Methyl ester of Naphthalene acetic acid MCPA : 2 Methyl 4 chloro phenoxy acetic acid TIBA : 2, 3, 5 Tri iodo benzoic acid 2 , 4-D : 2, 4 dichloro phenoxy acetic acid 2 , 4, 5-T: 2, 4, 5 – Trichloro phenoxy acetic acid Auxins are of two main types

9. Indole-3-acetic acid (IAA), the most important natural auxin in plants, is mainly synthesized from the amino acid tryptophan ( Trp ). the amino acid tryptophan ( Trp ) is a pre cursor of IAA as it has a close similarity with it and is presumably present in all cells. The progress in auxin biosynthesis also lays a foundation for understanding polar auxin transport and for dissecting auxin signaling mechanisms during plant development. Biosynthesis of Auxin

10. The Indole-3-pyruvic acid (IPA) pathway : In this pathway, first of all the amino acid tryptophan donates its amino group to another α- keto acid by transamination reaction to become indole pyruvic acid. The reaction is catalyzed by enzyme indole tryptophan transaminase. Then indole pyruvic acid undergoes decarboxylation in the presence of enzyme indole pyruvate decarboxylase to become indole acetaldehyde. Finally indole acetaldehyde oxidizes to become indole-3-acetic acid. This reaction is catalyzed by enzyme indole acetaldehyde dehydrogenase. Examples- sterile pea shoots, in cucumber seedlings, etc. IAA biosynthesis pathways in plants

11. Tryptophan Transaminase Indole-3-pyruvic acid IPA Decarboxylase Indole-3-acetaldehyde Dehydrogenase Indole-3-acetic acid (IAA) IPA - PATHWAY

12. The Tryptamine (TAM) pathway: Tryptamine occurs sporadically in higher plants. It was first isolated from Acacia and since been found in several other species. In this pathway Tryptophan is decarboxylated to form Tryptamine in the presence of enzyme tryptophan decarboxylase, followed by deamination to form indoleacetaldehyde in the presence of enzyme tryptamine oxidase. Indoleacetaldehyde is then oxidized to form Indole-3-acetic acid in the presence of enzyme indoleacetaldehyde dehydrogenase. Cont …

13. TAM - PATHWAY Tryptophan Decarboxylation Tryptamine Amino oxidase Indole-3-acetaldehyde Dehydrogenase Indole-3-acetic acid (IAA)

14. The Indoleacetaldoxime (IAN) pathway: This pathway is characteristic of the family Cruciferae . In this, tryptophan is converted into Indole-3-acetaldoxime which in the presence of Indole-3-acetaldoxime hydrolase converts it into indole-3- acetonitrile (IAN). The enzyme nitrilase coverts Cont …

15. IAN - PATHWAY Tryptophan Tryptophan mono oxygenase Indole-3-acetaldoxime Indole-3-acetalnitrite Nitrilase Indole-3-acetic acid (IAA)

16. Went (1928) reported that auxin is transported basipetally , i.e. it moves from apical to basal end. The movement is quite fast, about 1 to 1.5 cm/h ( in roots 0.1 to 0.2 cm/h). McCready and Jacobs (1963) working on petiole segments of Phaseolus vulgaris observed acropetal movement of auxin . Acropetal movement of auxin consumes metabolic energy whereas basipetal movement is purely a physical process which goes on even under anaerobic condition. Auxin Translocation

17. Recent studies have indicated that auxin transportation in plant is an overall active transport system. Some points in favour of it may be summarized as fellows :- The velocity of auxin transport (1 to 1.5 cm/h) in stems and coleoptiles is about ten times faster than diffusion. ( ii) Metabolic inhibitors are less effective on auxin transportation as compared to poisons. 2,3,5- triiodobenzoic acid and naphthylthalamic acid are highly effective. ( iii) Auxin transportation depends upon aerobic metabolism. (iv ) Auxins are able to move against a concentration gradient. ( v) It has been observed that there is good correlation between structural requirements for auxin activity and the auxin transport. ( vi) The saturation effect of transport site being in between 0.1 and 1 µM per tissue volume also supports it. Cont …

18. In general, transport of auxin is effected by temperature, oxygen, gravity, age, applied chemicals etc. Auxin is transported through the living cells including phloem parenchyma and parenchyma cells that surrounds the vascular bundle. The transport is inhibited by antiauxins , e.g. 2,3,5- triiodobenzoic acid (TIBA), naphthylthalamic acid (NTA) and ethylene chlorohydrin. DCA ( dichloroanisole ) is anti- auxin effective against 2,4-D. Cont …

19. Auxin is destructed by the enzyme IAA oxidase in the presence of O2 by oxidation . IAA Oxidase IAA + H2O2 + O2 3-methylene oxindole + H2O + CO2 Rapid inactivation may also occur by irradiation with x-rays and gamma rays. UV light also reduces auxin levels in plants. Inactivation or decomposition of IAA by light has been called as photo oxidation. Destruction / Inactivation of auxin in plants

20. Cell Elongation and longitudinal growth: The growth of cells in length is called cell elongation. The primary and chief function of auxin in plants is to stimulate the cell elongation in shoot. Extensive studies on the mechanism of auxin action in cellular elongation shows that auxins promote cell elongation by; causing increased wall plasticity with decreased elasticity, enhancing the water and solute uptake and interacting at gene level by synthesizing enzymes that are required for the synthesis of cell wall and cytoplasmic components. Auxins induce enzymatic or non-enzymatic deformation or loosening of cell wall by breaking the cross links between the cell wall components. This results in increased plasticity with decreased elasticity of the cell wall. The bonds are reformed after the cellular elongation. Physiological Role of Auxins

21. The influence of apical bud in suppressing the growth of lateral buds is called apical dominance. If the terminal bud is intact and growing, the lateral buds remain suppressed, particularly in long and sparsely branched vascular plants. Removal of apical bud results in the fast growth of the lateral buds. The reason of this was explained by Thimann and Skoog (1934) and Thimann (1937). According to them, the auxin is synthesized in the apical meristem from where it is translocated downwards causing inhibition of growth of lateral buds. During downward movement of auxin some correlative inhibitors are synthesized which inhibit the growth of lateral buds. Recent investigations support the view that auxins induce the formation of ethylene which acts as inhibitor for lateral buds. Apical Dominance

22. Auxins initiate as well as promote cell division in tissues like cambium. This effect of auxin is particularly important in secondary growth of stem and differentiation of xylem and phloem Cell division

23. The same concentration of auxin does not show the same effect on the different parts of plant. Sometimes the conc. of auxin which increases growth of one organ, reduces or inhibits the growth of other organ, e.g. the conc. of auxin which accelerates the growth of stem, reduces the growth of roots but the number of lateral branches in roots is increased. Auxins always inhibit root growth at higher concentrations. At low concentrations, they promote root growth. The conc. of auxin which is inhibitory to root growth causes initiation of adventitious roots from the nodes or basal regions of stem. The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots. Root growth and root initiation

24. Natural auxins control the falling of fruits, flowers and leaves from the plants. The leaves and fruits fall down from the plants only when abscission layer is formed between petiole or pedicel or fruit stalk and stem at the point of attachment. This region is called abscission zone. It has been shown that abscission zone does not occur when the conc. of auxin is high, it always occurs when auxin gradient becomes less or neutral. It is also believed that the hormone ethylene promotes the abscission (due to auxin induced synthesis of ethylene). Prevention of abscission

25. The formation of seedless fruits without fertilization is called Parthenocarpy . Although it is common in nature but parthenocarpic fruits can be obtained treating the flowers with low conc. of auxins . Parthenocarpy has been induced successfully by synthetic auxins in fruits like orange, banana, apple, tomato, pineapple, cucumber, etc. Parthenocarpy

26. Auxins stimulates respiration in many plants and there is a close correlation between auxin induced growth and an increased rate of respiration. It is believed that auxin increases the supply of ADP from ATP by utilizing the latter in the expanding cell. The increase in the availability of ADP increases the rate of ATP formation and thus stimulates respiration. Respiration

27. Auxins normally inhibit the flowering. However in litchi and pineapple auxins like 2,4-D and NAA have been found to promote flowering. When auxins in dilute concentrations are sprayed on pineapple plants, uniform flowering occurs. Low conc. of auxins as NAA or IAA are also effective in inducing flowering in barley. Flower initiation

28. Auxin helps in the reduction of size of branches. It is generally observed in plants like apple and pear where two types of branches, small and big, are found and fruits are born only on small branches Shortening of internodes

29. Auxin regulates phototropism, geotropism and other developmental changes. The uneven distribution of auxin , due to environmental cues, such as unidirectional light or gravity force, results in uneven growth of plant tissue. Generally, auxin governs the form and shape of the plant body, direction and strength of growth of all organs and their mutual interaction. Some other effects of auxins includes: wound response, in improving size and quality of fruit, as weed killer, etc. Plant growth movements

30. IAA increases the plasticity of cell walls so that the cells stretch easily in response to turgor pressure. It has been suggested that IAA acts upon DNA to influence the production of mRNA. The mRNA codes for specific enzymes responsible for expansion of cell walls. Recent evidences indicate that IAA increases oxidative phosphorylation in respiration and enhanced oxygen uptake. The growth stimulation might be due to increased energy supply and it is also demonstrated that auxin induces production of ethylene in plants. Mechanism of Action

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